The present invention relates to photoelectric object sensing apparatus, and more particularly to photoelectric object sensing apparatus of the type commonly referred to as light screens, in which a plurality of light beams and corresponding light detectors define a region of space where objects that interrupt or block one or more of the beams are sensed.
In accordance with the prior art, it is known to use light screen systems comprising a plurality of light beam transmitters and a plurality of corresponding light receivers, each positioned to receive a beam of light from a respective one of the transmitters, for sensing the presence of objects within a region of space between the transmitters and receivers. Such systems are commonly used in safety control equipment for dangerous machines, such as power operated presses and other automatic machines and robots, to actuate an alarm or to cause the stoppage of the machine when the presence of an object, such as an operator's hand, is sensed in an unsafe zone of the machine. Since the purpose of machine safety control equipment is to safeguard against accidents, light screen systems used in such equipment must be highly reliable and fail-safe. Ideally, the failure of any component which can result in a malfunction of the system should produce the same response as when an object is sensed by the system. Known light screen systems constructed entirely with hard-wired circuitry generally provide only a limited degree of safety from component failures.
In addition to being reliable and fail-safe, light screen systems used in machine safety control equipment must also have a high degree of immunity to spurious actuations caused by disturbances such as electrical transients, ambient light, vibration, aging of components, smoke or dust particles. Spurious actuations are highly undesirable in that they result in false alarms or unnecessary stoppages of the machine. In known light screen systems, immunity to spurious actuations can generally be achieved only at the expense of the sensitivity of the system to objects that are intended to be sensed. Consequently, in such systems there must be a compromise between reliability of sensing and immunity to spurious actuations.
Accordingly, a need exists for a highly reliable light screen system which provides a high degree of safety from component failures as well as a high degree of immunity against spurious actuations.